As much as possible, optimize grayscale images for the black channel only. This makes color control easier and reduces the final production cost. There are various ways to separate a color theme into grayscale, but the matter doesn't end there. Sometimes, separating colors is not that easy.
Imagine you have a full-color image, and a part of it needs to be converted to grayscale. Using the Grayscale color mode will convert the entire file to grayscale, but our main goal is to convert only a part of the file's subjects.
One technique that can create a grayscale area within a CMYK file is using the Gradient Map command. This process can simulate a color subject with the black channel. If you use commands like Hue/Saturation, the image might appear grayscale, but it will be present in all channels, which is very problematic.
Image > Adjustment > Gradient Map
- Use tools like the Lasso or Pen tool to select your subject. Now, run the Gradient Map command and define pure white and black at the first and last steps.
- Your subject will be transferred to the Black channel and become grayscale, while the rest of the file remains colored.